It is known to electrolytically reduce nitrate and nitrite from solutions of alkali metal hydroxides. This procedure is disclosed in U.S. Pat. No. 3,542,657 to Mindler and Tuwiner, the disclosure of which is incorporated herein by reference.
It has recently been found that this procedure is inoperative when the solution of nitrate and nitrite in sodium hydroxide is contaminated with oxidizing transition metal ions such as chromium ion. The chromium contamination which will give rise to this interference may be as low as 60 parts per million (as Na.sub.2 CrO.sub.4).
The reason for this interference is that in the electrolysis, nitrate is reduced to a nitrite at the cathode: EQU NO.sub.3.sup.- +2H.fwdarw.NO.sub.2.sup.- +H.sub.2 O (I)
However, the presence of chromate proximate to the cathode will reverse this reaction: EQU 2NO.sub.2.sup.- +CrO.sub.4.sup.-- .fwdarw.2NO.sub.3.sup.- +CrO.sub.2.sup.-(II)
at the anode the chromate is reoxidized: EQU CrO.sub.2.sup.- +2O.fwdarw.CrO.sub.4.sup.-- (III)
here, a trace of chromate will, as found, make the prior art process inoperative.
It is generally known that bismuth will react with chromate to form a water insoluble pearlescent pigment compound. When it was first found that the chromium interfered with the regular process of the prior art procedure, it was attempted to remove the chromium by reacting the same with bismuth ion. EQU 4CrO.sub.4.sup.-- .fwdarw.2Cr.sub.2 O.sub.7.sup.-- +O.sub.2 (IV) EQU O.sub.2 +2Bi.sup.+++ +Cr.sub.2 O.sub.7.sup.-- .fwdarw.(BiO).sub.2 Cr.sub.2 O.sub.7 (V)
The expected precipitation of the bismuth/chromium compound did not occur although a temporary operability of the process, for a very brief period of time, was noted.
It is furthermore known that when bismuth ion is subjected to electrolysis in an aqueous medium, black metallic bismuth will be formed on the cathode and purple/brown bismuth pentoxide (which is an insulator) is formed on the anode.